The present invention generally relates to nondestructive inspection methods and systems. More particularly, this invention relates to a method and system capable of nondestructively detecting and quantifying anomaly levels within materials, including composite materials such as a ceramic matrix composite (CMC) material.
Composite materials generally comprise a fiber reinforcement material embedded in a matrix material, such as a polymer or ceramic material. The reinforcement material serves as the load-bearing constituent of the composite material, while the matrix material protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. In CMC materials, reinforcement materials in the form of continuous or chopped fibers (filaments) may be coated with a release agent, such as boron nitride (BN) or carbon, to form a weak interface or de-bond coating that allows for limited and controlled slip between the fibers and the ceramic matrix material. As cracks develop in the CMC material, one or more fibers bridging the crack act to redistribute the load to adjacent fibers and regions of the matrix material, thus inhibiting or at least slowing further propagation of the crack through the matrix material. From this, it should be appreciated that the presence of porosity in CMC and other composite materials not only affects the strength of the matrix material, but also the ability of the reinforcement material to carry and distribute loads within the composite material.
CMC materials reinforced with continuous fibers, often referred to as continuous fiber reinforced ceramic composites (CFCC), offer light weight, high strength, and high stiffness, and are therefore of particular interest to a variety of high-temperature load-bearing applications, including shrouds, combustor liners, vanes, blades, and other high-temperature components of gas turbine engines. The continuous fibers may be arranged to form a unidirectional array of fibers, or bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven or braided to form a three-dimensional fabric. Also of particular interest to high-temperature applications are silicon-based CFCCs that employ silicon carbide (SiC) as the matrix and/or reinforcement material. A notable example of a silicon-based CFCC has been developed by the General Electric Company under the name HiPerComp®, and contains continuous silicon carbide fibers in a matrix of silicon carbide and elemental silicon or a silicon alloy. Particular examples of SiC/Si—SiC (fiber/matrix) CFCC materials and processes are disclosed in commonly-assigned U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898, 6,258,737, 6,403,158, and 6,503,441, and commonly-assigned U.S. Patent Application Publication No. 2004/0067316. One such process is known as “prepreg” melt-infiltration (MI), which in general terms entails the fabrication of CMCs using multiple prepreg layers, each in the form of a tape-like structure comprising the desired reinforcement material, a precursor of the CMC matrix material, and one or more binders. Multiple prepreg layers are stacked and debulked to form a laminate preform, a process referred to as “lay-up,” followed by curing while subjected to applied pressure and an elevated temperature, such as in an autoclave. The laminate preform is then heated (fired) in a vacuum or an inert atmosphere to convert the precursor to the desired ceramic material, decompose the binder, and produce a porous preform that can then be melt infiltrated with molten silicon or another suitable infiltrant during the same or subsequent heating step. The infiltrant fills the porosity and, depending on its composition, may react to form additional matrix material.
As previously noted, the presence of any residual porosity is an important issue since it can have a significant affect on the properties of a CMC material. It should be well understood that other types of void defects, such as delaminations, are also detrimental to the strength of CMCs and other composite materials. Consequently, various nondestructive examination (NDE) and nondestructive test (NDT) techniques have been considered for determining void levels in CMC materials, including but not limited to immediately following a melt infiltration process. In addition to being sensitive to the volumetric void level, the properties of a CMC material can differ depending on the type of void, for example, a discrete anomaly, generalized porosity, or delamination. Consequently, NDT methods capable of detecting and characterizing void conditions are needed. Current NDT methods for composites include radiography (RT), ultrasonic (UT), computed tomography (CT) scanning, and infrared (IR) thermography such as flash infrared thermography (Flash IR) and through transmission thermography (TT IR). Results of these NDT methods routinely require the analysis of a trained technician.
In practice, Flash IR imaging techniques have been found to work well for near-surface indications with high levels of porosity. However, Flash IR produces a two-dimensional (2-D) image, and the detection capability of Flash IR decreases as part thickness increases and as the pore size and percentage of porosity decrease. Though TT IR overcomes some of the limitations of Flash IR, the result is still a 2-D image. As a result, though an estimate of percent porosity can be made from the IR data, it can be difficult to determine the size (depth and thickness) of the porosity or other void indication. Without such information, the full impact of a void on material properties cannot be known.
UT and RT (X-ray) techniques have certain advantages and disadvantages over IR when used to inspect various composite material systems, though both are 2-D imaging technologies and therefore share the same limitation as IR in terms of being incapable of fully characterizing the depth and thickness of a detected void condition. In contrast, CT scanning generates a full three-dimensional (3-D) image of a test specimen by utilizing digital geometry processing of a series of two-dimensional X-ray images taken around a single axis of rotation. However, the generated 3-D data set is very large and must be reviewed by a trained technician. To fully assess the percentage of porosity, a technician would require hours to evaluate the CT data set and characterize the porosity indications.
In view of the above, it would be desirable if a fast and reliable nondestructive test method existed for detecting and characterizing void regions within a composite material to ensure material properties.